Method for making cardiac leads with zone insulated electrodes

ABSTRACT

An electrode for a cardiac lead and method of making the same are provided. The electrode includes an electrode member and a coating applied to the electrode member. A method of fabricating a high impedance cardiac lead electrode is provided. The method includes the steps of providing an electrode member and coating a first portion of the electrode member with an electrically insulating material and placing a tubular mask or shield over the electrode. Portions of the insulating material are removed to expose selected areas of the electrode. The second or exposed portion enhances the impedance of the electrode, resulting in power savings and extended life spans for implantable stimulation and sensing devices. Exemplary materials for the coating includes diamond-like carbon and sapphire.

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/092,106, filed Jun. 5, 1998, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to cardiac stimulator leads, and moreparticularly to a cardiac stimulator lead having an electrodeselectively coated with an insulating material to define smallconductive regions.

2. Description of the Related Art

Conventional cardiac stimulator systems consist of a cardiac stimulatorand an elongated flexible cardiac lead that is connected proximally to aheader structure on the cardiac stimulator and is implanted distally atone or more sites within the heart requiring cardiac stimulation orsensing. The cardiac stimulator is normally a pacemaker, acardioverter/defibrillator, a sensing instrument, or some combination ofthese devices.

At the time of implantation, the distal end of a cardiac lead isinserted through an incision in the chest and manipulated by thephysician to the site requiring electrical stimulation with the aid of aflexible stylet that is removed prior to closure. At the site requiringelectrical stimulation, the distal end of the lead is anchored to theendocardium by an active mechanism, such as a screw-in electrode tip, oralternatively, by a passive mechanism, such as one or more radiallyspaced tines. The proximal end of the lead is then connected to thecardiac stimulator and the incision is closed.

Many implantable cardiac stimulators include a microprocessor or controlcircuit enclosed within a sealed housing or can. The circuit boardcontrols the delivery of electric pulses to the lead and may performvarious other functions. Power is supplied by an internal battery.

A conventional cardiac stimulator lead normally consists of anelongated, flexible, tubular, electrically insulating sleeve connectedproximally to a connector that is adapted to couple to the header of acardiac stimulator can and connected distally to a tubular tipelectrode. One or more ring-type electrodes may be secured to the sleeveat various positions along the length of the sleeve. The proximal end ofthe lead sleeve is connected to the connector by application of variousbiocompatible adhesives to various portions of the connector and thesleeve. The tip electrode ordinarily consists of a tubular structurethat has an increased diameter portion that forms an annular shoulderagainst which the distal end of the lead sleeve abuts. The exteriorsurface of the tubular structure is normally smooth as is the interiorsurface of the distal end of the lead sleeve. In multi-polar leads, oneor more ring-type electrodes may be fitted over the sleeve.

To ensure that physical contact with the desired myocardial tissue ismaintained after implantation, tip electrodes for most conventionalleads are anchored to myocardial tissue by a fixation mechanism of onesort or another. In some leads, a corkscrew-like member projects fromthe tip electrode and penetrates the endocardium. In others, theelectrode is fitted with one or more radially projecting tines thatengage the trabeculae within the heart. Still others may employ bothtypes of structures.

Most conventional tip electrodes serve at least two functions. First,tip electrodes provide a conducting member to convey electricalstimulation and sensing signals to and from myocardial tissue. Second,most tip electrodes provide structure to accommodate a fixationmechanism. Although conventional ring electrodes may be fitted withtines, most ring electrodes serve primarily as signal conductors.

The design of cardiac stimulation systems involves a balancing of anumber of competing design considerations. Some of these include cansize, lead tip dimensions and power consumption. Can miniaturization hasbeen an important design goal since the first implantable pacemakerswere introduced over thirty years ago. Smaller cans yield betterpost-operative comfort and cosmetic results for the patient. However,can miniaturization has required downsizing in storage batteries, whichhas, in turn, placed a premium on power consumption. Power consumptionis of great importance because for a given level of power consumption,smaller batteries generally translate into shorter cardiac stimulatorlife spans and more frequent surgical procedures for the patient.

Some of the limitations associated with diminishing battery size havebeen offset by advances in cell chemistry. In addition, advances inpulse generation circuitry have dramatically increased the efficiency ofpower consumption. For example, many cardiac stimulators incorporatecircuitry that automatically tailors pulse generation to thephysiological demands of the patient.

However, despite advances in battery chemistry and circuitry, powerconsumption efficiency is still frequently limited by conventional leadelectrode design. Most conventional lead electrodes operate asrelatively low impedance, and thus, high current drawing devices. Thelow impedance levels are primarily a function of the relatively largeconducting surface areas that these devices present to myocardialtissue. As noted above, the size of conventional lead electrodes isdictated in large part by mechanical considerations, such as thefacilitation of fixation mechanisms. Furthermore, a certain degree ofbluntness in a tip electrode is desirable to reduce the risk ofmyocardial perforation and micro-dislodgement, and to facilitate captureof the lead tip by post-implant developing fibrous tissue. Similarly,miniaturization of ring-type electrodes is generally limited by the sizeof the insulating lead sleeve and by the prevailing mechanical systemsused to secure such ring-type electrodes to the lead sleeve.

As a result of these mechanical design considerations, current is oftendrawn by conventional low impedance electrodes at higher rates thannecessary for appropriate stimulation. Some improvement in current drainmay be realized by lowering the voltage output of the pulse generator.However, this technique is not possible in patients who require athreshold voltage for successful stimulation that is above thecontemplated lowered output voltage. Thus, conventional lead electrodedesigns may represent an impediment to extended battery life.

In one conventional lead design, the distal end of the lead is providedwith a distally projecting, small diameter circular electrode that hasthe potential to provide enhanced pacing impedance. However, this designmay be prone to micro-dislodgment. Since the lead is provided with asingle small conducting surface on the distal end of the lead, normalheart motion may cause the small conducting surface to momentarily losecontact with or micro-dislodge from myocardial tissue and disrupt theflow of pacing pulses.

The present invention is directed to overcoming or reducing the effectsof one or more of the foregoing disadvantages.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method of fabricating a highimpedance cardiac lead electrode is provided. The method includes thesteps of providing an electrode member and coating a first portion ofthe electrode member with an electrically insulating material andplacing a tubular mask or shield over the electrode. Portions of theinsulating material are removed to expose selected areas of theelectrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings in which:

FIG. 1 is a pictorial view of an exemplary embodiment of a cardiacstimulator lead and a cardiac stimulator in accordance with the presentinvention;

FIG. 2 is an exploded side view of an exemplary cardiac lead electrode,sleeve and conductor in accordance with the present invention;

FIG. 3 is an end view of the electrode shown in FIG. 2 in accordancewith the present invention;

FIG. 4 is a cross-sectional view of FIG. 2 taken at section 44 inaccordance with the present invention;

FIG. 5 is a cross-sectional view like FIG. 4 showing the electrode priorto coating with an insulating material in accordance with the presentinvention;

FIG. 6 is an exploded side view like FIG. 2 of an alternate exemplaryelectrode in accordance with the present invention;

FIG. 7 is an end view of the electrode depicted in FIG. 6 in accordancewith the present invention;

FIG. 8 is a perspective view of a tubular shield being prepared for usein the method of the present invention;

FIG. 9 is a perspective view of the prepared shield of FIG. 8;

FIG. 10 is a cross sectional view of an electrode tip with shield; and

FIG. 11 is a perspective view of a tubular shield being prepared for usein an alternative fashion.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In the drawings described below, reference numerals are generallyrepeated where identical elements appear in more than one figure.Turning now to the drawings, and in particular to FIG. 1, there is shownan exemplary cardiac stimulator lead 10 that includes a flexibleinsulating sleeve 12 that has a proximal end 14 coupled to a connector16, and a distal end 18 coupled to a tip electrode 20. The connector 16is designed to be inserted into a cardiac stimulator 22, and is shownhighly exaggerated in size relative to the cardiac stimulator 22. Thecardiac stimulator 22 may be a pacemaker, a cardioverter/defibrillator,or other type of stimulator or a sensing instrument. The illustratedembodiment of the lead 10 is bipolar. Accordingly, the distal end 18 isprovided with an electrode 24 located proximal to the tip electrode 20.However, unipolar or other multi-polar arrangements are possible aswell. A suture sleeve 26 is slipped over the sleeve 12. Duringimplantation, the suture sleeve 26 is sewn to body tissue at the site oftransvenous entry.

The sleeve 12 is a flexible tubular member that provides a robust,electrically insulating coupling between the connector 16 and theelectrode 20. The sleeve 12 protects one or more fine gage conductorwires enclosed therein from body fluids and tissues, and isadvantageously composed of a biocompatible, electrically insulatingmaterial, such as silicone, polyurethane, or like materials.

The detailed structure of the electrode 20 may be understood byreferring now also to FIG. 2, which is an exploded side view of theelectrode 20 and the end 18 of the sleeve positioned distal from theelectrode 24, and to FIG. 3 which is an end view of FIG. 2. Theelectrode 20 includes an electrode member 28 that has an elongatedmandrel-like shank 30 that is provided with a set of external grooves orthreads 32 at its proximal end 34 and terminates in an enlarged diametertip 36. The grooves 32 may be formed integrally with the shank 30 ormachined as a separate structure that may be welded or otherwiseconnected to the shank 30. The transition from the shank 30 to thelarger diameter tip 36 defines a proximally facing annular shoulder 38.The tip 36 has a profile that tapers inwardly to a circular blunt orflat end surface 39. Although the profile of the tip 36 is largely amatter of design discretion, an overall blunt profile of the distal endof the tip 36 reduces the potential for myocardial penetration andmicro-dislodgment.

The electrode member 28 is advantageously fabricated from abiocompatible conductor or semiconductor material. Suitable materialsinclude, for example, iridium oxide coated titanium, MP35N, stainlesssteel, platinum-iridium alloy consisting of approximately 90% platinumand 10% iridium, or some other biocompatible conducting metal, or asemiconductor material, such as silicon, or other semiconductormaterial. A portion of the electrode 20 may be composed of other than aconducting material so long as a conducting pathway is provided betweenthe conductor wire 40 and the tip 36.

A conductor wire 40, shown exploded from the electrode 20, is slippedover the proximal end 34 of the shank 30 and spiraled around the grooves32 when the lead 10 is assembled. The wire 40 is depicted as a coiledmetallic conductor wire that is individually insulated with a thininsulating jacket. An end 42 of the wire 40 is stripped as shown toestablish a good electrical contact with the exterior of the shank 30.The end 42 may also be spot welded by laser or other suitable techniquesto the exterior of the shank 30. The proximal end of the wire 40 iscoupled to the connector 16 shown in FIG. 1. A second conductor wire(not shown) is nested with the conductor wire 40 and is coupled distallyto the annular electrode 24 and proximally to the connector 16, and ispositioned in a nested arrangement with the wire 40 within the sleeve12. The skilled artisan will appreciate that other wiring arrangementsmay be incorporated in lieu of the individually insulated wire 40 andthe companion wire (not shown). For example, commonly used coaxialwiring arrangements may be incorporated where the individual wire coilsare separated by an inner elongated tubular insulating sleeve.

When the lead 10 is fully assembled, the distal end 18 is slipped overthe shank 30 until a distally facing annular shoulder 44 on the distalend 18 abuts the proximally facing annular shoulder 38 of the tip 36. Asuitable medical grade, biocompatible adhesive may be applied to theexterior of the shank 30 and/or the interior of the distal end 18 tosecure the distal end 18 to the electrode member 28. The adhesive may bea silicone based adhesive, or one of a variety of commercially availabletwo stage biocompatible adhesives.

As noted above, a low impedance electrode in a cardiac lead can resultin power consumption that is beyond the rate necessary for medicallyindicated cardiac stimulation and/or sensing. Although power supplydepletion is inevitable in self-contained cells, unnecessary powerconsumption represents a real limit on battery life. However, inaccordance with the present invention, the electrode 20 may befabricated with a higher impedance than would otherwise be possible inview of the conducting nature and structural requirements of theelectrode 20. A lead fitted with the electrode 20 in accordance with thepresent invention may reduce power consumption and prolong battery lifefor the cardiac stimulator 22 without sacrificing stimulation and/orsensing functions.

The impedance enhanced character of the electrode 20 may be understoodnow by referring to FIGS. 2, 3, 4, and 5. Relative to FIG. 2, FIG. 3 isan end view, and FIGS. 4 and 5 are sectional views taken, respectively,at sections 4--4 and 5--5. A first portion 45 of the exterior of theelectrode member 28 from the distal end of the grooves 32 to the end 39of the tip 36 is covered by a coating 46 composed of an electricallyinsulating material. A preselected second portion of the exterior of theelectrode member 28 consisting of six peripherally spaced, circularspots 48 on the tip 36 is re-exposed, as will be explained below. Thecoating 46 substantially reduces the otherwise available conductingsurface area of the electrode member 28. The exposed circular areas orspots 48 provide small conducting surfaces to contact and transmitelectrical current between the electrode 20 and myocardial tissue. Thereduced surface area of the electrode member 28 that may be exposed tomyocardial tissue dramatically increases the impedance of the electrode20, thus lowering the power consumption of the lead 10, and increasingthe operating life of the power supply for the cardiac stimulator 22shown in FIG. 1.

In the embodiment illustrated in FIGS. 2, 3, 4, and 5, the first portion45 of the electrode member 28 includes all of the exterior of theelectrode member 28, save the exposed areas 48, the grooves 32, and theproximal end 34. This configuration is illustrative as the desiredincrease in electrode impedance may be realized when the coating 46 isapplied to at least the portion of the electrode member 28 that will bein contact with myocardial tissue. The skilled artisan will appreciatethat enhanced impedance may also be achieved by covering a greater or alesser amount of the exterior of the electrode member 28. For example,the grooves 32 may also be coated if provision is made to establish aconducting connection between the stripped end 42 of the wire 40 and thegrooves 32. Conversely, the coating 46 may be applied only to theportion of the electrode member 28 that will contact myocardial tissue,i.e., the tip 36, exclusive of the proximally facing annular shoulder38.

The size, and configuration of the portion of the exterior of theelectrode member 28 that is exposed following application of the coating46 is largely a matter of design discretion and will depend on factorssuch as the electrical requirements of the cardiac stimulator and themedically indicated stimulation voltage, among others. For example, asshown in FIG. 6, which is a side view of an alternate embodiment of theelectrode, now designated 20', may be understood by referring now toFIGS. 6 and 7, which are, respectively, an exploded side view and an endview of the electrode 20'. In this embodiment, the tip 36 of theelectrode member 28 is provided with six peripherally spaced slots 50that commonly intersect a circular bore 52. The slots 50 divide the tip36 into a corresponding number of peripherally spaced projections 54.Each projection 54 has vertical sidewalls 58, 60, and 62.

Polymeric coatings, such as parylene compounds, may be applied using atool appropriate for the particular material. For example, Parylene Cmay be applied using a parylene vacuum deposition system which deliverspoly-para-xylylene into a vacuum chamber containing the targetedstructure, e.g., the electrode member 28.

After the polymeric coating has been applied to the electrode, selectedareas of the coating are removed to expose the conductive surface underthe coating. This is accomplished by applying an abrasive processthrough a mask or shield. Plasma etching is a suitable abrasive processwhere a parylene compound is used for the coating 46. A tubular sleeve70, having a proximal end 80 and a distal end 82, includes apre-selected pattern of openings 74 corresponding to the pre-selectedpattern of spots 48. The sleeve should be composed of a material thatwill withstand the removal process while protecting those portions ofthe coating 46 that are intended to remain intact. Preferably, the tubeis comprised of medical grade silicon rubber. As illustrated in FIG. 8,the sleeve 70 is prepared by placing it on an anvil 72 or support table.A punch 76 is driven through the sleeve to form a circumferential set ofholes or openings 74. The punch may be a tube having a beveled cuttingedge 78. Other shapes are, of course, feasible. Driving the punchthrough the sleeve 70 into the anvil 72 produces two diametricallyopposed holes. This method produces an even number of openings, forexample four or six openings, spaced around the circumference of thesleeve, as shown in FIG. 9.

After the holes 74 have been punched in the sleeve, the sleeve is placedover the tip electrode 20 as shown in FIG. 10. Heptane may be used toexpand the silicon tube slightly, making it easier to place the sleeveon the electrode tip. The shield is oriented so that the holes 74 arelocated over those areas where exposure of the conducting portion of theelectrode is desired. For example, in the hexagonally notched electrodetip illustrated in FIG. 10 and described in connection with FIG. 6 andFIG. 7 above, the holes could be placed adjacent each segment 54. Afterthe sleeve is placed over the tip, the distal end 82 of the sleeve isfilled with medical grade adhesive, forming a complete shield around thetip. The tip is then exposed to plasma from a plasma source 86 to abradethe polymeric coating. After the polymeric coating has been removed inthe desired locations, the sleeve is removed from the electrode tip.

It may also be desirable to provide a shield with an odd number ofholes, or with a series of holes that are not symmetrically spacedaround the circumference of the tube. For instance, it may be desired toprovide conducting areas on only three of the six segments 54 of theelectrode tip of FIG. 6. In such cases, a rod 88 may be temporarilyinserted in the sleeve 70, as illustrated in FIG. 11. Delron™ rods aresuitable for this purpose. The punch 76 will then cut a hole in only oneside of the sleeve at a time before being encountering the rod 88.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

What is claimed is:
 1. A method, comprising:providing an electrodemember; coating a first portion of the electrode member with anelectrically insulating material; providing a tubular shield, where thetubular shield has perforations at selected locations; placing saidshield over at least said first portion of said electrode member;etching said electrically insulating material exposed through saidperforated tubular shield; and removing said shield.
 2. The method ofclaim 1, wherein coating includes coating the first portion with theelectrically insulating material that comprises parylene C.
 3. Themethod of claim 1, wherein placing said tubular shield over at leastsaid first portion of said electrode further comprises filling a lumenof said tube with medical adhesive.
 4. The method of claim 1, whereinplacing said shield comprises expanding said tubular shield.
 5. Themethod of claim 4, wherein expanding said tubular shield comprisestreating said shield with heptane.
 6. The method of claim 1, whereinproviding the tubular shield includes perforating said tubular shield byboring circumferential through bores completely through said tubularshield.
 7. The method according to claim 6, wherein perforating saidtubular shield comprises placing a rod in a central lumen of saidtubular shield, punching holes through said shield to said rod, andremoving said rod from said tubular shield.
 8. The method of claim 1,wherein removing said shield comprises expanding said tubular shield. 9.The method of claim 8, wherein expanding said tubular shield comprisestreating said shield with heptane.
 10. The method of claim 1, whereinproviding the electrode member includes providing the electrode membercoated with iridium oxide.